Barney, I'm interested in hearing your report as well.

Totin_Tools if your mileage is failing right now, installing an overdrive box will only band-aid a gaping wound. Nothing wrong with an OD unit at all, I am saving up for a US gear for myself.
But you need to determine the cause of you mileage and fix that first or along with the installation of an OD box.
The common causes of poor mileage on the 7.3's seems to be leaking turbo up-pipes, clogged exhaust pressure sensor pipes and/or failing sensors themselves and small fuel leaks on the drain valve for the filter canisters.
I cleaned out my exhaust pressure sensor and pipe, it made no difference.
I resealed my turbo up-pipes and gained 2.5 mpg when empty. Towing it made no difference, but I am not surprised at that. I have a rather high profile trailer and it just takes a certain amount of fuel energy to ram that big trailer through the atmosphere.

 

Kwikk, I just got a new pressure tube and it seemed like I got some power back that I might have lost. I realize it takes a certain amount of power to move our rigs, but I have been losing mpg's for a while now. I need to check the up-pipes like you have said and am planning on doing that today. If it doesnt rain. I am just thinking of ways to try to save some money in fuel prices. I dont have a 5er I just have an enormous tag along. On the up-pipes did you just get new donuts to reseal them or did you use some kind of compound?

 

I replaced my up-pipes about 3 days before the bellowed pipes came out.
I highly recommend spending the extra money on a set of those and putting them in. You will never have to do that job again, and I am looking forward to redoing mine in about 20,000 miles.

If the budget simply won't allow that, then go get some donut gaskets and replace them. I used a lot of copper based anti-sieze as a lubricant for all the parts. The heat of the exhaust dried it it pretty quickly, but so far no black marks anywhere and the turbo keeps the boost up at the slightest provocation.

 

Ok, I'm calling to find some gaskets right now. The passenger side is leaking but not the drivers. That passenger side is a real pain, cant see anything and all. Thanks.

 

bfife, I reread the thread and was going to suggest the possibility of installing an external cooler. But I saw that Gene mentioned this as well. Is it feasable to do. Just thinking out loud. But of course the body would have to have an outlet & inlet port and be able to cycle the fluid on its own. I don't remember the configuration of the OD units myself. Could this work? Country

 

Excellent idea!!!

 

Thanks Dan. I got to thinking about it alittle more as well and I don't see why a temp sensor/gauge couldn't be installed to monitor the temps, to see what OD unit is doing! Ahh well I guess if I had one I would try it and report on it. But I don't have a GV unit. Maybe, if I put it on my Christmas list? I better be nice to Momma!! HEE HEE..

 

Kwik, I can't give you a report yet, the darn things are still on factory BO.,

maintenance is very simple, just a fluid change, the fluid is expensive, GM synthetic manual gear oil only.

You cannot install an outside cooler. there are no external ports and the pump is driven by the input shaft. it doesn't start to work until you get up to about 20 mph.

the Gear Vendor cannot be left on with a exhaust brake. they won't stand to have reverse torque applied. pressure from the rear wheels will cause them to fail.it cannot be used in 4wheel drive.

when they fail, the internal discs wear out. if you look at one you will see a dark gray/black band on the outside. this is the friction band. when they overheat the gaskets will push out and they will begin to leak. at this point they are toast.however
GearVendor will replace them for 1/4 original cost.

the US gear doesn't care if you have a exhaust brake, they can be used in 4x4. they are built similar to a manual tranny. steel gears and shift forks. slightly larger fliud capacity also.

The GearVendor is easier to install. It goes behind the tranny/tranfer case and you only need to shorten one drive shaft.
The Us Gear unit goes between the Tranny and Tcase. you have to lengthen the front and shorten the rear.
the electronics are very similar on both units

The Gear vendor is rated at 30,000 lbs if you use it with an automatic tranny and are not a commercial user or fulltime RVr. with a 2 year warranty.
if you are a commercial user,fulltime Rvr or have a manual tranny, GearVendor shortens you warranty to 1 year and weight limits to 25,000 lbs.

US gear doesn't care what you do or what you drive, 35,000 lbs period. I don't remenber if its 1 yr or 2. Doesn't matter, I've got one on order.

 

US gear all the way, then. Kind of like comparing a bulldog to a poodle.

 

HAH HAH HAH. YUP I'd say that about sums it up Dan. bfife, thanks for the great info too. Very Informative. Country

 

I would've gotten back sooner, but the WiFi has been down, but that's not nearly as bad as that bridge going down just south of us here in Duluth, and I've still got to cross over the big "M" when I head west.

The dully comes stock with 235 85R 16 tires which are also 31.6" diameter like my 265 75R 16 tires, at least I'm pretty sure they are. This is the best way to measure your tire, so I can tell for sure if you've got tranny slippage or not. Put a chalk mark where the tire tread touches the road on both the tire tread and the road, go forward until the mark hits the road, and measure the linear distance along the road with a tape, and give me that number, and I'll compare that to the calculated circumference using 31.6" for the diameter, and figure your exact tire diameter that way. Make this measurement several times to make sure you're getting the same # each time, or report all the #'s, and I'll average them.

The reason I suspect possible tranny slippage is because if your tire diameter is 31.6", as I think, then taking (64.6/60)4.10=4.41 and (71/60)3.73=4.41, which as you'll see in the table below, are two ways to calculate your diff ratio based on your measured 60 mph at 2000 rpm. If you look in the table below at 2000 rpm you see a 4.10 gives 64.6 mph, and a 3.73 gives 71.0 mph. If I did a table for a 4.41 diff (which doesn't exist) it would show 60.0 mph at 2000 rpm. A 4.30 diff (which is possible) gives 61.6 mph at 2000 rpm, and a smaller diameter than 31.6" with a 4.30 diff could give 60 mph at 2000 rpm. That's why you need to measure your rear tire, and also give me what your Speedo reading is at 2000 rpm.

I would bet that you've got a 4.10 diff, because that's normal in a dully, and that over time you've developed an increasing amount of slippage in your TC, which has caused your mpg to decrease.

The table below is for mph vs rpm for both a 4.10 (blue) and a 3.73 (red) for each gear of an auto tranny. Note, this table is for the TC locked solid with no slippage, and is like for a manual with these same gear ratios. When the TC is unlocked, the rpm can be much higher at a given mph than is shown in the table due to the slippage in the TC.

As you can see, the table uses an overdrive ratio input, and I put this together years ago when I was analyzing what I'd get with different GV or US Gear units. An overdrive ratio of 0.909756 turns a 4.10 into a 3.73. When I analyzed the 0.8 ratio that's actually offered, it didn't seem to fit well with my 3.73 diff, so I just wound up as changing to a 4.10 diff instead.

 

Ok Eugene, I will go and measure my tires and get that back to you. I have also thought about just pulling the cover off the diff to see what is stamped on the ring and pinion. Thanks for all your engineering skills you are happy to share with all of us.

 

No need to pull the diff cover, but it never hurts to change the fluid and clean it out inside once in awhile. You can jack up the rear end, grip the drive shaft with a pipe or a strap wrench if necessary, and count how many revs it takes to turn the rear wheel 1 rev. First put the tranny in neutral!

EDIT... and release the parking brake!

 

Ok I did find out that I infact have 4.10 gears. Went to the driveline place here by the house. I since have measured my tires and I found them to be 31" exactly. So maybe the 0.6" shorter height is where my problem lies at, with the rpm at 2000 and 60 mph. Maybe when I replace these tires I will get some taller ones. I'm gonna call the tire shop and find out how many revs at 60 these are doing. At least I think thats the speed their book says.

 

Below is a table for a 31.00" tire diameter, and at 2000 rpm for your 4.10 diff it gives 63.4 mph (blue) vs your measured 60.0 mph. The only possible explanation I can come up with to explain this discrepancy (63.4 vs 60.0) is that your lock up torque converter is going bad and slipping some, and it's gotten a little worse over time which is also why your mpg has decreased over time.

You should be able to test my theory by getting up to 55- 60 mph on a flat road (no problem there), and watching both your tac and Speedo as you apply the throttle, and compare those #'s with the ones in the table I provided. If the RPM is increasing to a progressively higher value than in the table for a given mph, as the load is progressively increased with the throttle, then that indicates the TC is slipping under load.

You can do a slip test better if you lived in an area with a long grade, because as you apply more throttle to pull a grade at a constant 2000 rpm, the mph should not change, no matter how much load you apply. If the mph starts to decrease at a constant rpm as the load increases, that indicates tranny slippage.